Methods and structures for preparing single crystal silicon wafers for use as substrates for epitaxial growth of crack-free gallium nitride films and devices

ABSTRACT

This document describes the fabrication and use of ceramic stabilizing layer fabricated right on the product silicon wafer to facilitate its use as a substrate for fabrication of gallium nitride films. A ceramic layer is formed and then attached to a single crystal silicon substrate to form a composite silicon substrate that has coefficient of thermal expansion comparable with GaN. The composite silicon substrates prepared by this invention are uniquely suited for use as growth substrates for crack-free gallium nitride films, benefitting from compressive stresses produced by choosing a ceramic having a desired higher coefficient thermal expansion than those of silicon and gallium nitride.

This application is a continuation of and claims priority from U.S.patent application Ser. No. 14/251,634, filed on Apr. 13, 2014, entitled“Methods and structures for preparing single crystal silicon wafers foruse as substrates for epitaxial growth of crack-free gallium nitridefilms and devices”; which is incorporated herein by reference.

BACKGROUND

Single crystal gallium nitride is a technologically important materialfinding increasing use in high frequency RF devices, and Light EmittingDiodes (LEDs). In the absence of methods to form single crystals of thisand similar materials from melt, they are invariably grown byhetero-epitaxy by metal-organic chemical vapor deposition, M-O-CVD, orby atomic layer deposition, ALD, on single crystal substrates ofsapphire (Al2O3), or silicon carbide (SiC), because of their refractorynature, purity, inertness, and reasonably close lattice structure matchto gallium nitride. Both sapphire and silicon carbide are in themselvesextremely hard to grow as single crystals, the larger the diameter, theharder to make them. Until recently, nearly 90% of gallium nitridecrystals were grown on 2-inch diameter. Only in 2009 this percentagedropped below 50%, and now most new LED fabricators are using 4″substrates, and some even venturing into 6″ diameter sapphire wafers.Growing GaN on single crystal silicon carbide is somewhat easier becauseof closer lattice matching, but silicon carbide wafers are stuck at 2″diameter. GaN growth, on the small diameter sapphire wafers entails anenormous loss of productivity. This is a great impediment to themaffordable for replacing the incandescent lighting. It is for thisreason that there has been a continuing effort to use silicon wafers assubstrates for GaN Epitaxy.

If silicon wafers can be used easily for growing gallium nitride, theadvantages of larger wafer sizes, wide availability, atomically smoothgrowth surfaces, would quickly lead to their wide adoption. Why is thisnot the case? Growing GaN epitaxially on silicon (111) would face both alarger lattice mismatch (17%), and a larger thermal expansion mismatch(about 50%). Researchers have been able to bridge the lattice mismatchthe same way as is done in cases of sapphire and silicon carbide, hereusing buffer layers of AlGaN to grow low defect GaN films on silicon.This greatly reduces the lattice strain in GaN layer and, as a result,reduces the dislocation density to reasonable levels. However, the signand magnitude of thermal contraction mismatch between GaN and silicon,are such to give rise to extensive cracking of the latter upon cooling.In practical terms, this limits the thickness and size of usefuldevices, and the yield of such devices.

Some ingenious methods for growing GaN on silicon have been developed toenable the use of silicon substrates for GaN growth. Almost all thesemethods are based on modifying the growth surface with a) use ofmultiple or varied buffer layers, b) limiting the size of crystalsgrowing and of crack prorogation by scoring the silicon wafer surface,c) limiting growth surface with in-situ silicon nitride masking, andallowing for lateral growth over the masked areas to fill the surface,and d) to change the growth morphology to nano rods. Even with thesedifficulties, after years of development, limited commercial productionof GaN on silicon substrates has just begun.

The one case where a silicon wafer was modified on the non-growth sidemissed the mark. They attached very thin silicon 111 wafer, or very thinsingle crystal silicon carbide wafer, to polycrystalline silicon carbidewafers, apparently to reduce cost of the growth wafers. They missed themark in the sense, that the support wafer bonded to the growth wafer,either had the same or similar coefficient of thermal expansion tosilicon, in one case, and silicon carbide, in another, to make anydifference in the cracking behavior. Even then, the researchers reportedgrowing good quality GaN epitaxial layers on 2″ substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B illustrate exemplary composite substrates according to someembodiments of the present invention.

FIGS. 2A-2B illustrate an exemplary process flow for forming a compositesubstrate according to some embodiments of the present invention.

FIGS. 3A-3B illustrate an exemplary process flow for forming a compositeGaN substrate according to some embodiments of the present invention.

FIG. 4 illustrates an exemplary flowchart for forming a compositesubstrate according to some embodiments of the present invention.

FIG. 5 illustrates another exemplary flowchart for forming a compositesubstrate according to some embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In some embodiments, this invention relates to preparing single crystalsilicon wafer for epitaxially growing gallium nitride, which can preventthe cracking that generally occurs in the deposited gallium nitridefilm, upon cooling from the elevated growth temperatures. Moreparticularly the invention teaches a new approach that involvesmodifying the silicon (111) wafer substrate, on the side opposite to thegallium nitride growth side, by coating of a suitable material having athermal expansion higher than that of GaN. This layer, hereinafterreferred to as a ‘stabilizing layer’, induces compressive stresses inboth the silicon and the GaN layers, upon cooling from room temperature.Since the induced compressive stress permeates into the GaN layer,though the silicon wafer, it in effect neutralizes the tensile stressinduced in the GaN by silicon and, thereby, stabilizes it.

In some embodiments, the silicon substrate is first prepared byintegrally attaching, or forming the stabilizing layer, in a separatestep prior to using it for epitaxial deposition of gallium nitride. Atthe GaN growth step, the epitaxially deposited GaAlN or similar bufferlayers, commonly known in the art, are first deposited on the (111) Sisurface to bridge the lattice mismatch to GaN, followed by thedeposition of GaN layer. Upon cooling to ambient temperature, the GaNlayer will remain intact without cracking. The composite wafer can beprocessed for device making and forming interconnections, after which itcan be diced, and separated from silicon layer by etching off thelatter.

In some embodiments, the stabilizing layer can be a refractory material,such as a thin film of metal or ceramic, thick film metal or ceramic, orbulk substrates of metal, ceramic or glass integrally bonded to thesilicon wafer surface. It is preferred that the material of thestabilizing layer have a coefficient of thermal expansion, CTE, higherthan that of GaN. The silicon wafer can be in pre-stressed conditionafter the application of the stabilizing layer, which generally requiresa higher temperature deposition for the layer. The thickness of thestabilizing layer can be such as to induce compressive stress in thesilicon upon cooling from the layer application temperature. In fact,the compressive stress can be sufficiently high also to inducecompressive stress in the gallium nitride after its deposition on thesilicon wafer, and cooling to ambient temperature. The stress level is afunction of the nature of the coating material; the process used for itsapplication, in particular the application temperature, its CTE, andthickness. Thinner silicon wafers are preferred, because thinnerstabilizing layers can be used to obtain the desired effect on thedeposited gallium nitride. These parameters can be modeled from knownthermal, mechanical and physical properties of silicon, the stabilizinglayer, and gallium nitride film. It is much more easily determined, atthe outset, from well designed experimentation.

FIGS. 1A-1B illustrate exemplary composite substrates according to someembodiments of the present invention. In FIG. 1A, a composite substrate100 is shown, comprising a silicon layer 110 disposed on a support layer120. In some embodiments, the silicon layer 110 comprises a singlecrystal silicon layer. The silicon layer can be less than 500 micronthick, preferably less than 100 microns, and more preferably is between10 and 50 microns. In some embodiments, the silicon layer is preferablyhave a (111) crystallographic surface. In some embodiment, an adhesivelayer 115 is disposed between the silicon layer 110 and the supportlayer 120, for example, to strengthen the bond between the two layers.In some embodiments, the support layer has coefficient of thermalexpansion (CTE) higher than that of GaN. Also, the effective CTE of thecomposite substrate 100 is higher than that of GaN so that asubsequently deposited GaN layer on the silicon substrate 110 does notcrack at room temperature.

In some embodiments, the thickness of the support layer is higher thanthat of the silicon layer. For example, the thickness of the supportlayer can be thicker than 100 microns, preferably thicker than 500microns, and can be thicker than 1 mm.

In some embodiments, the support layer and the silicon layer arestrongly bonded together, and thus acting as a composite substrate withrespect to thermal expansion. The effective thermal expansion of thecomposite substrate is a result of a balance between the high thermalexpansion of the support layer and the low thermal expansion of thesilicon. The effective coefficient of thermal expansion can becalculated from the thermal expansion of the composite substrate. Ingeneral, the coefficient of thermal expansion is related to theindividual coefficients of thermal expansion and thicknesses of thesupport layer and the silicon substrate. For example, a much thickersupport layer will provide an effective coefficient of thermal expansionsimilar to that of the support layer, since the effect of the siliconlayer is smaller.

In FIG. 1B, a composite substrate 150 is shown, comprising a GaN layer130 disposed on a silicon layer 110 disposed on a support layer 120. Thesubstrate 150 can be formed by depositing a GaN layer on a substrate110. In some embodiments, the thickness of the GaN is higher than 2microns, and can be higher than 5 microns. The effective CTE of thecomposite substrate 150 is higher than that of GaN so that the GaN layerdoes not crack after cooled to room temperature. In some embodiments,the GaN layer comprises a buffer layer under a GaN layer for latticematching with the silicon layer 110.

While the present description utilizes single crystal siliconsubstrates, other substrates can be used, such as silicon-containingsubstrates (e.g., SiGe substrates, composite substrates having a siliconlayer on a support substrate, etc.).

Without limiting the scope of this invention the types of stabilizinglayer can be categorized into several categories, viz. (i) vacuumdeposited thin films of metal or ceramic, (ii) thick films (of metal,ceramic, or glass), (iii) bulk substrates (of metal, glass, or ceramic)with an attachment layer of glass or metal, (iv) in-situ formedglass-ceramic coatings.

(i) Depositing Stabilizing Layers:

In some embodiments, highly stressed films can be deposited, forexample, at the backside of the silicon substrate. If the stress of thecomposite substrates (e.g., silicon substrates having the depositedfilms) is sufficiently high, it can compensate for the stress induced bythe GaN when cooling, and the stress of the GaN would be compressive. Interms of thermal expansion, if the thermal expansion of the compositesubstrates is higher than that of the GaN, this would prevent crackingin the GaN upon cooling, with the stress of the GaN compressive.

In some embodiments, sputtered or evaporated thin films of refractorymetals such as molybdenum and tungsten, if deposited at elevatedtemperatures, can induce large compressive stress in silicon. Thisprocess would be a natural choice in the semiconductor processingculture, if the selected metals are not detrimental to the subsequentprocesses, e.g., do not introduce contamination to the GaN layer (or tothe devices that form on the silicon substrate), for example, reactingwith the process gases, the silicon substrate, and gallium nitride. Theeconomics of this approach depends on the thickness of films required toachieve the desired level of beneficial compressive stress in thegallium nitride layer later grown on the silicon wafer. The advantagesof this method are clean processing, ease of thickness control, goodthermal conductivity of metal films, and compatibility with downstreamdevice processing.

In addition to metals, other material, such as ceramic films depositedby a sputtered process, can also be used to induce the desired stress inthe silicon substrate. The thickness of such sputtered ceramic filmswould need to be optimized, since thin films with inadequate thicknessmight not be able to withstand the balancing tensile stress and thesubsequently deposited gallium nitride would crack upon cooling.

(ii) Coating Stabilizing Layers:

In some embodiments, alternating to deposition processes, such as vacuumthin film deposition processes, other coating processes can be used toform a composite substrate that can sustain the device fabricationprocesses, for example, the high temperature deposition of GaN.

In some embodiments, thick films are formed by applying to substratepowders of ceramics, glass, or metals, in the form of a spray-ableslurry, or printable paste (such as metal inks) with suitable organicbinders and solvents, followed by heating to expel the organics, andsinter the powder to produce bulk coatings on the substrate. To improvethe adhesion of the consolidated ceramic or metal powders to theunderlying substrates, an adhesive can be used, such as mixing a glasspowder with the ceramic or metal powder. The glass powder would melt andhelp consolidate the ceramic powder and also bond to the oxidizedsilicon surface. The adhesive would need to be optimized to preventinterfering with the subsequent device processing. For example, certainglass powder exhibits temperature softening in the glass phase, and thuslimiting the refractoriness in the formation of the stabilizing layer,which can be a serious limitation for using thick film stabilizinglayers for typical gallium nitride growth temperatures.

Other criterions would also need to be considered in the material andprocess selections for forming the thick film coating, such as metalcontamination, softening, and compatibility with downstream deviceprocessing.

(iii) Bulk Substrates for Stabilizing Layer:

In some embodiments, bulk metal, such as stainless steel, molybdenum,tungsten, and bulk ceramic substrates of polycrystalline aluminum oxide,aluminum nitride, zirconia, can be used as the support layer. Adhesionadditives can be used, since metal substrates and ceramic substratesmight be separated at high temperature. Exemplary adhesion materialsinclude a glass or metal bonding layer at the interface. The same istrue for glass substrates such as those made of Pyrex or similarrefractory glass. Limitations in size and cost for these substrates area further factor to be considered.

(iv) In-Situ Formed Glass-Ceramic Coating or Substrates for StabilizingLayer:

In some embodiments, powders of certain glass compositions, when heatedto temperatures in the range of the softening point of the correspondingbulk glass, crystallize and densify to yield essentially a ceramic bodymore refractory than the parent glass. If these glass powders aresuitably disposed on a suitable substrate, such as a silicon wafer, theywould also adhere well to the substrate during such consolidation. Thisprovides a convenient method for forming self-adhering, refractorystabilizing layer of this invention on the silicon wafer, provided theCTE of the resulting glass-ceramic is higher than the CTE of galliumnitride. A convenient method to dispose the glass powder is to firstform a green tape of the glass powder by mixing it with suitablepolymeric binders and plasticizers and solvents. The green tapetechnology is already well developed for fabricating so called LowTemperature Co-fired Ceramic, or simply LTCC substrates. Details of theLTCC substrates have been disclosed in co-pending patent applicationSer. Nos. 12/558,486 and 12/558,490, hereby incorporated by reference.

A wide choice of glass compositions are known in the literature with thedesired sintering and crystallizing characteristics. These glasscompositions have been developed for fabricating low temperatureco-fired ceramic, or simply LTCC, substrates. In our preferred approach,glasses in the MgO—Al₂O₃—SiO₂ system, having MgO in the range of 15-28%by weight, Al₂O₃ in the range of 9-15% by weight, the remainder made ofsilica, except for less than 2% of other ingredients such as TiO₂, ZrO₂,P₂O₅, or B₂O₃. The glass powders of these compositions fully densify inthe temperature range of 850° C. to 950° C., and yielding dense, strongglass-ceramics having thermal expansion coefficients in the range of4-6.5 ppm/° C., higher that of both silicon and gallium nitride, asdesired.

In some embodiments, to form the stabilizing layer of this invention,the green ceramic tape of the suitably chosen glass powder is applied tothe single crystal silicon wafer, typically of (111) orientation, on theside opposite to the polished side reserved for later gallium nitridedeposition. The green tape is placed on the silicon surface and heatedto temperatures of 50-100° C., and at pressures of 500-1000 psi designedto soften and stick it securely to the wafer surface. The wafer-greentape assembly is then cured at the required high temperature. Duringthis consolidation the glass sinters to a strong and dense,glass-ceramic body, strongly bonded to the oxidized silicon surface. Oncooling from the consolidation temperature, the differences in thethermal coefficients expansion, CTEs, of silicon and the resultingglass-ceramic will induce significant compressive stress in the siliconand corresponding tensile stress in the glass-ceramic stabilizing layer.The glass-ceramic layer should be sufficiently strong to resistcracking, and sufficiently thick to avoid excessive wafer bow.

This pre-stressed silicon is then cleaned and prepared for galliumnitride epitaxial deposition. At the deposition temperature, the wafercomposite will be essentially stress-free and flat. As the wafer iscooled to ambient temperature, compressive stresses develop in bothsilicon and the gallium nitride layers, preventing cracking that wouldhave otherwise occurred in the absence of the glass-ceramic stabilizinglayer. The composite wafer is processed as needed to form discretegallium nitride layers. The wafer is then diced, and devices arereleased by etching off the silicon.

Ceramic coatings for silicon wafers of this invention are fabricatedfrom ceramic precursors suitably disposed on one of the major surfacesof the silicon wafer and heating in air to temperatures in the range of800-1200 C, and more preferably to between 900-1100 C, expel the organicbinders and consolidate the ceramic powder into bulk ceramic coatings,whose CTE will be in the range of 6-10×10 ppm/C. of GaN measured fromits growth temperature of around 1000 C. Examples of suitable ceramicprecursors include refractory ceramic powders, such as aluminum oxide,zirconium oxide, mullite, mixed with suitable glass powders that fuseduring the first heating step to bind the ceramic powders to form a bulkceramic coating having CTE equal to or greater that of gallium nitride

In some embodiments, the glass powders can comprise glass compositionsin systems in MgO—Al₂O₃—SiO₂, CaO—Al₂O₃—SiO₂, BaO—Al₂O₃—SiO₂, ormixtures thereof, which when heated to temperatures in the preferredtemperature range of 900-1100 C, fuse and crystallize to form a bulkcoating having CTE>CTE of GaN.

A composite structure consisting of a continuous (crack-free) singlecrystal gallium nitride layer greater than 2 micron thickness, singlecrystal silicon and polycrystalline ceramic coating having CTE higherthan that of gallium nitride as measured from its (GaN's) growthtemperature, in that order.

In some embodiments, gallium nitride layer may comprise gallium nitridelayer may consist of suitable buffer layers aimed at bridging themismatch in lattice between that of silicon and gallium nitride. Galliumnitride layer may consist of suitable gallium nitride alloy layers forelectronic device fabrication. In some embodiments, electronic devicescan be fabricated on the GaN layer, such as light emitting diodestructures.

FIGS. 2A-2B illustrate an exemplary process flow for forming a compositesubstrate according to some embodiments of the present invention. InFIG. 2A, a slurry of glass powder 220 is disposed on a silicon substrate210. The slurry can be sprayed, or pasted on the silicon substrate. Theslurry can comprise glass powder, polymeric binders, plasticizers, andsolvents.

In FIG. 2B, the silicon substrate with the slurry layer is heatedsolidify the slurry, forming a composite substrate comprising a siliconlayer 210 disposed on a ceramic layer 225.

FIGS. 3A-3B illustrate an exemplary process flow for forming a compositeGaN substrate according to some embodiments of the present invention. InFIG. 3A, a composite substrate comprising a silicon layer 210 disposedon a ceramic layer 225 is provided. The composite substrate can beprepared by attaching a ceramic layer on a silicon layer, such as in aprocess described above. In FIG. 3B, an optional buffer layer 260 isdeposited on the composite substrate, followed by a GaN layer 270. Thebuffer layer 260 can serve as a lattice matching layer, to match thelattice of GaN with that of the silicon.

In some embodiments, the present invention discloses methods for forminga continuous (or crack-free) gallium nitride layer on siliconsubstrates. An exemplary method of forming a single crystal galliumnitride layer of thickness exceeding 2 microns on a single crystalsilicon substrate can comprise the steps of

a. Forming a polycrystalline ceramic layer having CTE greater than thatof gallium nitride on one of the planar sides of a single crystalsilicon substrate in a first heating step

b. Cooling the ceramic-coated silicon substrate to ambient temperature

c. Growing single crystal gallium nitride layer of thickness exceeding 2microns in thickness on the silicon surface opposite to the ceramiccoating on the silicon substrate in a second heating step

d. Cooling the composite of single crystal gallium nitride layer

e. Fabricating GaN nitride on the composite gallium nitride layer(optional)

In some embodiments, a ceramic precursor is disposed on the siliconsubstrate before the first heating step. After the first heating, theceramic precursor solidifies and bonds with the silicon substrate toform a composite substrate. In some embodiments, forming step ofpolycrystalline ceramic layer having CTE equal to or greater that ofgallium nitride including forming from ceramic powder precursors in thefirst heating step. The first heating step can be carried out totemperatures in the range of 800-1200, and most preferably totemperatures in the range of 900-1100 C.

In some embodiments, the ceramic precursors include refractory ceramicpowders, such as aluminum oxide, zirconium oxide, mullite, mixed withsuitable glass powders that fuse during the first heating step to bindthe ceramic powders to form a bulk ceramic coating having CTE equal toor greater that of gallium nitride. The ceramic precursors can includecertain glass powders, such as from glass compositions in systems inMgO—Al₂O₃—SiO₂, CaO—Al₂O₃—SiO₂, BaO—Al₂O₃—SiO₂, or mixtures thereof,which when heated to temperatures in the preferred temperature rangefuse and crystallize to form a bulk coating having CTE>CTE of GaN. Thesingle crystal silicon can be of (111) orientation. In some embodiments,the single crystal silicon surface is from (100) orientation, and whichis then completely covered with pyramids of having (111) facets producedby anisotropic etching.

In some embodiments, growing gallium nitride step includes epitaxiallydepositing single crystal gallium nitride from pyrolitic decompositionof certain gallium metal organic gaseous precursors in MOCVD reactor athigh temperatures. In some embodiments, growing gallium nitride stepincludes first depositing buffer layers aimed at bridging the latticesof silicon and of gallium nitride to be grown thereon. The buffer layerscan include aluminum nitride, aluminum gallium nitride, zirconiumfluoride, and others known in the art. Gallium nitride layer can includegallium nitride alloy layers required to form suitable devicestructures. For example, the alloy layers can include magnesium dopedgallium nitride layers, or silicon doped gallium nitride layers. In someembodiments, gallium nitride devices including light emitting diodes(LED).

FIG. 4 illustrates an exemplary flowchart for forming a compositesubstrate according to some embodiments of the present invention.Operation 400 provides a single crystal silicon substrate, wherein thesilicon substrate comprises a first side and a second side. Operation410 attaches a layer to the first side of the silicon substrate to forma composite substrate, wherein the layer has coefficient of thermalexpansion (CTE) higher than that of GaN, wherein the effective CTE ofthe composite substrate is higher than that of GaN so that asubsequently deposited GaN layer on the second side of the siliconsubstrate does not crack at room temperature. In some embodiments, themethod further comprises applying an adhesive to the first side of thesilicon substrate before attaching the layer.

In some embodiments, the second side of the silicon substrate comprisesa (111) crystallographic surface. The thickness of the silicon substratecan be less than 50 microns. The attaching the layer can comprisedepositing the layer in vacuum. The layer can comprise a refractorymetal or ceramic. The attaching the layer can comprise bonding a bulkmetal layer or a bulk ceramic layer to the silicon substrate through anadhesion layer. The attaching the layer can comprise spraying a slurryon the silicon substrate and sintering to bond the slurry with thesilicon substrate, wherein the slurry comprises a mixture of a powder ofceramic, glass, metal, or a combination thereof. The slurry can comprisean adhesive additive. The attaching the layer can comprise pasting apaste on the silicon substrate sintering to bond the paste with thesilicon substrate, wherein the slurry comprises a mixture of a powder ofceramic, glass, metal, or a combination thereof. The paste can comprisean adhesive additive. The attaching the layer can comprise disposing aglass powder to the silicon substrate and sintering the glass powder toform the composite substrate.

FIG. 5 illustrates another exemplary flowchart for forming a compositesubstrate according to some embodiments of the present invention.Operation 500 provides a single crystal silicon substrate, wherein thesilicon substrate comprises a first side and a second side. Operation510 mixes glass powder with polymeric binders to form a ceramicmaterial, wherein the glass powder has coefficient of thermal expansion(CTE) higher than that of GaN. Operation 520 disposes the ceramicmaterial to the first side of the silicon substrate to form a compositesubstrate. Operation 530 sinters the composite substrate to bond theceramic material with the silicon substrate. Operation 540 deposits alayer of GaN on the second side of the silicon substrate of the sinteredcomposited substrate, wherein the effective CTE of the sinteredcomposite substrate is higher than that of GaN so that the deposited GaNlayer does not crack when cooled to room temperature. In someembodiments, a buffer layer is deposited before depositing the GaNlayer. In some embodiments, the ceramic material comprises a magnesiumoxide-aluminum oxide-silicon oxide composition. The sintering comprisesannealing at temperature 850 to 950 C.

What is claimed is:
 1. A method comprising providing a single crystalsilicon substrate, wherein the silicon substrate comprises a first sideand a second side; applying a paste on the first side of the siliconsubstrate; sintering the paste to solidify the paste, wherein thesolidified paste has coefficient of thermal expansion (CTE) higher thanthat of silicon.
 2. A method as in claim 1 wherein the second side ofthe silicon substrate comprises a (111) crystallographic surface.
 3. Amethod as in claim 1 wherein the thickness of the silicon substrate isless than 50 microns.
 4. A method as in claim 1 further comprisingapplying an adhesive to the first side of the silicon substrate beforeapplying the paste.
 5. A method as in claim 1 the paste comprises arefractory metal, ceramic, a powder of ceramic, glass, metal, or amixture thereof.
 6. A method as in claim 1 wherein the paste comprisesan adhesive additive.
 7. A method as in claim 1 wherein the sinteringprocess is between 850 and 950 C.
 8. A method as in claim 1 wherein thesintering process is between 800 and 1200 C.
 9. A method as in claim 1wherein the silicon substrate and the solidified paste form a compositesubstrate, and wherein the effective CTE of the composite substrate ishigher than that of GaN.
 10. A method comprising providing a singlecrystal silicon substrate, wherein the silicon substrate comprises afirst side and a second side; placing the silicon substrate in adeposition chamber; depositing a layer on the first side of the siliconsubstrate in vacuum, wherein the layer has coefficient of thermalexpansion (CTE) higher than that of silicon.
 11. A method as in claim 10the layer comprises a refractory metal, ceramic, glass, metal, or amixture thereof.
 12. A method as in claim 10 wherein the siliconsubstrate and the deposited layer form a composite substrate, andwherein the effective CTE of the composite substrate is higher than thatof GaN.
 13. A method comprising providing a single crystal siliconsubstrate, wherein the silicon substrate comprises a first side and asecond side; applying a slurry on the first side of the siliconsubstrate; sintering the slurry to solidify the slurry, wherein thesolidified slurry has coefficient of thermal expansion (CTE) higher thanthat of silicon.
 14. A method as in claim 13 wherein the thickness ofthe silicon substrate is less than 50 microns.
 15. A method as in claim13 further comprising applying an adhesive to the first side of thesilicon substrate before applying the slurry.
 16. A method as in claim13 the paste comprises a refractory metal, ceramic, a powder of ceramic,glass, metal, or a mixture thereof.
 17. A method as in claim 13 whereinthe slurry comprises an adhesive additive.
 18. A method as in claim 13wherein the sintering process is between 850 and 950 C.
 19. A method asin claim 13 wherein the sintering process is between 800 and 1200 C. 20.A method as in claim 13 wherein the silicon substrate and the solidifiedslurry form a composite substrate, and wherein the effective CTE of thecomposite substrate is higher than that of GaN.